Chicks love it, because bad boys are always appealing, and the sensitive part builds rapport. Average schlub guys like it, because they think 'Hey, I'm somewhat vulnerable and introspective too! Maybe women will like me now!' (They won't).

Add in some catchy pop/rock instrumentation, and it's a winning formula. Clearly Robbie Williams understands vulnerability game. Which sounds about right, given he seems to have pretty good game in general.